Caridean shrimps are an integral component of lowland river ecosystems in south-eastern Australia, but their distributions may be affected by flow alteration. Monthly shrimp samples were collected from slackwaters in three hydrologically distinct sections of the heavily regulated Campaspe River and the less regulated Broken River for three consecutive years. The distributions of Paratya australiensis, Caridina mccullochi and Macrobrachium australiense, along with their life history in river sections with different hydrology are outlined. Paratya australiensis and M. australiense occurred in all sections, but C. mccullochi was absent from sections of the Campaspe River that received irrigation flows during summer/autumn. Shrimp larvae were most abundant in summer (December–February) and juvenile recruitment continued through to mid autumn (April). Breeding and recruitment of P. australiensis occurred for longer than other shrimps. Apart from large adult and berried M. australiense, all life stages of shrimps commonly occurred in slackwaters, particularly the larval and juvenile stages. Irrigation flows in summer/autumn probably adversely affect the size, extent and arrangement of slackwaters, at a time when they may be critical habitats for C. mccullochi larval development and recruitment. Dams and weirs in the Campaspe River may have influenced shrimp abundance and the timing of breeding.

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The dynamic nature of habitat patches in rivers is driven primarily by flow regime. Altered hydrology, through river regulation, can limit the size and distribution of slackwater patches; important areas for the development of young fish and for shrimp in lowland rivers. Between late October 2002 and late January 2003, we investigated responses of fish, shrimp and their potential prey to the experimental creation of slackwaters and, conversely, to the experimental creation of flowing patches, by diverting water away from flowing patches and into slackwater patches, respectively. A pre-experimental survey indicated that slackwaters contained many more fish than flowing patches, and fish larvae were flushed out of slackwaters during the construction of flowing patches. Creation of slackwaters resulted in increased abundance of fish and shrimp, with the opposite occurring when slackwaters were changed into flowing patches. Converting slackwaters into flowing patches, and vice versa, altered the species composition of zooplankton and microbenthic assemblages but did not change their densities. Thus, standing crop of potential prey alone could not explain the differences in fish or shrimp abundance found between patch types. We hypothesize that slackwaters primarily act as refuges from current and provide energetic advantages to the young stages of fish and to shrimp. River regulation has the potential to affect the recruitment success of fish and shrimp by affecting the size, arrangement and availability of slackwater patches.

We present and test an extension of the "match/mismatch" hypothesis that attempts to explain the persistence, under conditions of flow alteration, of small, short-lived, native, riverine, fish species. The premise is that flow alteration typically changes environmental conditions, such as temperature and prey abundance, which may affect survival during the larval period of fishes. This "window-of-opportunity hypothesis" states that, if optimal conditions for recruitment vary temporally within a year, the probability that a proportion of the larvae of protracted-spawning species will encounter a period of optimal conditions is greater than for larvae with only a brief spawning period, and so the former will have a recruitment advantage. We determined whether all hatching events contributed equally to juvenile recruitment of the protracted-spawning Australian smelt (Retropinna semoni) during one breeding season in three pairs of heavily regulated and largely free-flowing unregulated rivers in the Murray-Darling Basin, Australia, and related patterns in the hatch dates of recruits to temperature or prey biomass for one pair. For all rivers, heavily regulated or not, recruits present at the end of the breeding season most commonly hatched in the latter part of the breeding season. Mortality of those fish hatched in the first part of the season likely explains this trend. Furthermore, while hatching times were similar for all rivers, each river showed a distinct pattern of hatching and recruitment, which may relate to the temperature range within which epigenetic processes are aligned. Patterns of zooplankton biomass differed between the largely free-flowing ovens and regulated Goulburn rivers and likely had different sources: within the channel and within the storage lake, respectively. For the Ovens River, recruits hatched subsequent to the period when the first significant increase in zooplankton biomass occurred. We hypothesize that temperature may largely influence the "window" during which recruitment can take place but that prey density, responding to river-specific interactions between temperature and discharge, plays a role in the timing and magnitude of recruitment of Australian smelt. We conclude that the match/mismatch hypothesis may be applicable to rivers, that the window-of-opportunity hypothesis has some currency and deserves further investigation, and that river regulation may have significant impacts on fish recruitment.

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